The invention relates to heat recovery steam generators (HRSG's). While the invention has particular application to recovering heat from the exhaust of a gas turbine used to generate electric power, it will be understood that the invention has other applications particularly where a fluid stream exhausting from a given process contains oxygen to support combustion.
Gas turbines have been widely used to generate electric power. The thermal efficiency of gas turbines is low because of high exit-gas temperatures (800.degree. to 1000.degree. F., or 425.degree. to 540.degree. C.) and high excess-air levels (220 to 300 percent) in the combustion products. The thermal energy remaining in the exhaust gas can be recovered in a heat-recovery boiler to produce additional electricity using a steam-turbine generator. The combined output of electricity from the gas turbine and the steam turbine is 30 to 50 percent greater than that obtained from the gas turbine alone, with no additional fuel input. Other heat recovery steam generators may include supplementary firing.
Supplementary fired heat-recovery steam generators use firing equipment located in the exhaust gas stream in the boiler inlet transition duct. Since gas-turbine exhaust contains 75 to 80 percent of the oxygen normally found in atmospheric air, fuel may be burned without the need for additional fresh air. By using duct burners (or other similar burners), gas-turbine exhaust temperatures can be increased to 1500.degree. to 1600.degree. F. (815.degree. to 870.degree. C. supplementary firing generally doubles steam output of the heat recovery boiler by providing a mechanism for varying steam production and matching process steam demand, independent of the gas turbine electricity production.
Boilers that are substantially assembled in a shop and in shipped to site for installation and operation, sometimes called "package boilers" have relatively low overall installed costs. Standardization is the key to reduced costs. Standardization of design saves time and money prior to actual fabrication of a shop assembled unit. Even greater savings are achieved in manufacturing. Standard shop assembled boilers are built under controlled conditions which permit a high quality product at low cost. The assembly area is arranged for optimum materials flow both of parts fabricated for individual units and for parts and materials to be taken from stock. No delays are encountered due to adverse weather conditions. Site preparation is minimal since most units require only a concrete slab foundation. No errection space or material storage space is required at the site.
Known package boilers typically have a single flow path for the products of combustion passing out of the gas turbine. Various heat exchanging surfaces are serially disposed in the single flow path. The total flow capacity for a given pressure drop across the boiler has a finite limit. Often this limit is not sufficiently large for many applications. The size of the package boiler cannot be increased to increase capacity because the maximum size is limited by the transport apparatus. For example, the size is limited by the size of available trucks, rail cars, and barges. The conventional package boiler does not lend itself to use as a heat recovery steam generator because the conventional structure will excessively restrict flow of exhaust gases from the gas turbine. Ordinarily, the logical alternative to is to build a larger boiler at the operating site that is much larger and much more expensive.
A primary object of the invention is to utilize a know package boiler with it's inherent advantages and to adapt the construction of a known package boiler to the requirements of a heat recovery steam generator.
It is also an object of the invention to modify the construction of a known package boiler to increase the gas flow capacity through the boiler so that the boiler does not restrict the flow of gas out of the gas turbine with which it cooperates.
A related object of the invention is to permit the use of a given package boiler for applications that would ordinarily require the construction of a larger package boiler or the construction at the site of a larger boiler at much greater expense.
Another object of the invention is to utilize substantial parts of the known package boiler to attain the efficiencies in design and manufacturing inherent in such known designs.
Still other object of the invention is to provide apparatus that will have good thermodynamic properties.